If you want to get into Marxism (even if it's just to learn about what people are actually talking about), Principles of Communism by Engels and How Marxism Works by Chris Harman are fantastic pamphlets that really take no time to read through, though beware, Harman is a Trotskyist and that bleeds through a bit in his writing.

Marx mostly spoke about Capitalism and while no Marxist can avoid reading Marx, he doesn't provide a great introduction to Socialism in the Marxist sense, if that makes sense. Still, Value, Price, and Profit and Wage Labor and Capital are fantastic intros to the critique of Capitalism.

Even if you're interested in learning about Marxism-Leninism, jumping straight to Lenin before even understanding Marxism would be a mistake. Lenin builds his own critique off of Marxism, as a Marxist, so it is preferable to go through Marx first.